I purchased this 1985 Corvette 3 wks ago. It was stored 25 years, well maintained and looks like new and has 60K on it. The idle fluctuates from 400 to 850 RPM at times and when scanned it is running very lean and will not stay in closed loop. The timing is fluctuating whenever it goes into the iirratic idle which does not always occur. Most of the time the idle is consistant although still lean. Have checked for vac leaks at engine and accessories and have found none. When single distributor connector is disconnected it idles evenly. ECU seems to be functioning normally. Mass air flow sensor appears to be working fine. Ran 2 tanks of fuel with Castle inj cleaner. Has anyone else had into this condition or have any ideas?

 

Start with the basics. What is your fuel pressure? Base timing? And how did you ascertain that there aren't any vacuum leaks or "false air"?

Also...sitting for 25 years?? I'd be pulling the fuel rails/injectors, cranking it over and watching for injector spray patterns. My guess is that some are at least partially clogged.

 

Congrats on the car. This is a common problem. Although I have never found a complete fix, my surging has reduced dramatically after three things. 1) minimum idle reset procedure. - you can search the forum. but need to follow instructions carefully on resetting the changed idle (check timing too). 2) intake manifold gasket replacement (had vacuum leak) and 3) new injectors.

I think my EGR is suspect but I have not replaced.

 

to both of you, are your 85’s autos?

 

Mine is 4+3

 

Had the erratic idle symptom on my 93 tbi chevy truck.. It was a bad 02 sensor and cheap fix!

 

Yep.. mine is an auto.

 

for your 85 auto, contact @Nomake Wan and buy an hlh3200 prom for your 85. install that and do the minimum idle set procedure and take care of all vac leaks.

if the hlh3200 prom is installed and your sensors are good then u will have a car that idles and iperates as good as the later years.

any 85 autos that do not have an hlh3200 chip in them, you dont know what u r missing!!

in late late late 85, gm released the hlh3200 for owners that complained about the rediculous low tcc lockup and the idle that just had a very hard time sorting itself out. the hlh3200 solves ALL THIS.

onviously the rest of the input sensors have to be functioning but if the car runs and starts fine with just a hard to dial in idle, you probably have a small vac leak and that combined with the pre hlh3200 chips makes for a horribly running 85.

With the 85, for scanning, you probably noticed when you scan it, it jumps to 1000rpm idle. long story short, the 85 cannot be “scanned” in its “normal” mode to diagnose certain issues. Due to the 85 being forced into “field test mode”, a variety of parameters are force-locked. rpm, some timing, other stuff.

So, you can see what the ecm sees, and what your o2 sees, and your tps, you will be able to see the sensors, but you cannot for example see what the iac is at while at “idle”. you will nitice timing gets manipulated, iac usually goes wide open, etc.

So scanning the 85 certainly isbuseful but not until the 86 can you truly get into fuel trim analysis etc. you can see it, but its not in “normal operating “ conditions that exist when the issues ate presented.

so before you think your icm is screwed up, and your knock sensor is broken, realize that only some things can be “viewed” reliably when scanning the 85.

in summary, with an 85 auto, if you are having runability issues, (including what may feel like “stack shifting” which is really just the tc locking up too soon as it was programmed by gm to fit emmisions....later rectified by hlh3200 for the squeaky wheels!)

- verify all your ecm input sensors are in spec, most but not all can be viewed by scanning if you can
- check fuel pressure and for leakdowns after shutoff
- carry out minimum idle test, good rule of thumb is if you cant kill the engine by fully backing out the tb plates, you have a good size vac leak! this can be an incorrectly sized iac as there is an early and late toi part number with different sized pintkes and the typical parts counters are not aware and cross supply the numbers, been there. follow the min idle set procedure and use 550 as minimum idle for the 85. gm updated from 450, to 500, then later to 550.
- grab an hlh3200 and install it and enjoy your 85.



————.

i have not owned an 85, 4+3 but something i have always wanted to try is to get someone like @Nomake Wan to graft in all the runability code from the hlh3200 which is for an auto, and make it work for the 4+3.

I discovered all of this with the help @WVZR-1 years ago. whenever i see 85 auto owners, i take the time to pass this knowledge along. It may be possible that the 4+3 does not have any idle runability issues. not sure havnt owned one.

for 85 autos, you will really enjoy the car with the hlh3200 and the factory installed 2200stall torque converter

 

Excellent info. Thank you.

 

I have a 1985 Z51 Auto... I had idle issues and here is what I did.
1) Check your manifold vacuum with a gage.
2) Make sure your throttle body is clean. You need to remove it to clean. (mine needed a cleaning)
3) Make sure the TPS is set correctly (0.54V +_ 0.075V). (Mine was fine)
4) Check the IACV per the FSM. (mine was loose)
5) Set the minimum idle speed to 500 (for auto) per the procedure in the FSM, with one additional important step. Disconnect the timing connector. My FSM left this step out. I learned the hard way. FSM says 400 RPM for auto and 450 RPM for 4+3, but this is too low and there was an official bulleting from GM changing this to 500 RPM. (My idle needed adjustment)

After all this mine idles much better at 550-600 RPM.
Another poster mentioned a PROM upgrade they did in late 1985. I have heard of this, but my personal opinion, is that I would only do this as a last resort.

One other poster mentioned this, but you should be aware that for 1985 only (and maybe 84) scanners cause the ECM to go into a Field Service Mode and you cannot check idle speed with a scanner in place, as the ECM will force the idle to go to 1000 RPM. I learned the hard way on this as well. Also you cannot clear codes with a scanner on an 85.

 

keep in mind its not just idle issues that hlh3200 solves. it changes the tcc lockup speed to that of an 86+ car. prevents stack shifting (which is actually tc lockup between shifts!)

this is not a hack and not a fancy chip. its a legit gm chip. it made my 85 auto operate like my 87auto. way better.

 

No real magic in HLH 3200. It appears to be the last 1985 auto trans bin released for 1985 on 11/27/1985 (likely after production had already ended). 550 rpm idle speed, 20 degrees of spark advance at 600 rpm and below. TCC lock 41 mph. TCC unlock 39 mph. Covered by Service Bulletin 85-187.

HLP 3266 is pretty similar (with only 127 bytes different with respect to HLH), but was released on 12/03/1985 for 4+3 cars (strangely MK2 only). 750 rpm idle speed with 20 degrees advance at 800 rpm and below. Covered by Service Bulletin 85-186.

The flat spark advance should provide some idle stability assuming the revs stay at or below 600/800 rpm respectively.

Unfortunately, I don't have any other Y-body $1F bins to compare against.

If you're unable or unwilling to tune, these bins may offer a reasonable compromise while addressing some common complaint issues. Nothing says quality like a re-release.

 

I replaced wires, plugs, rotor and cap, TPS, and IAC. Now the idle is rough and will not accelerate without misfire. I put the heat gun laser on each cylinder and two of the eight were over 200 degrees. I need to resolve my problem. I put the old TPS in, and my high idle was resolved. I checked all of the lines for vacuum, no vacuum leak. I had this same issue right after replacing the IAC and TPS only, including the pigtail/harness for both. Any idea for my situation?